The invention relates to an agricultural tractor, and more particularly, to a lifting structure which is attached thereto.
DE-A-28 04 129 shows an agricultural tractor, configured as a straddle tractor, wherein a chassis is supported through front and rear stilts on the front and rear wheels. The chassis carries a lifting arrangement that consists of a first frame, with vertical side pieces configured as rails, and a second frame, arranged to slide in the rails. The second frame carries a three-point hitch for implements, whose operating height can be controlled by two actuating cylinders arranged symmetrically between the first and the second frame. In any case, an operator-friendly adjusting capability for the three-point hitch is attained at a considerable construction cost.
A multi-purpose agricultural vehicle is also known from DE-C-28 07 299, whose vehicle superstructure is supported through leaf springs on an axle body which carries the rear wheels. In the vicinity of the leaf springs a pressurized fluid cylinder is connected in joints between the axle body and the superstructure whose pressure chamber can be connected through a shut-off valve to a hydro-pneumatic reservoir, acting as supplementary spring. Arrangements for the attachment of implements are not described in this document.
DE-A-31 07 228 describes a far-reaching standardization of the implement coupling, such as trailer attachments adjustable for height, low attachments for single-axle trailers with rigid or automatic coupling, three-point attachments, drawbars etc. In each such system consideration is given to the power-take-off drive and electric and hydraulic quick-coupling devices. By the use of an electronic lifting control the agricultural tractor with attached implement can be operated above 8 km./hour as an active system with dynamic vibration damping of the implement mass, and below 8 km./hour with drawbar pull regulation. However, the problem is that single-axle trailers generate a comparable vibratory excitation without being able to apply the electronic lifting control to their reduction, since the single-axle trailer is coupled to the trailer coupling arrangement and not to the three-point hydraulic hitch.
In connection with agricultural tractors, recent efforts have been directed at reducing the various coupling arrangements to their basic functions: carrying, pulling, driving, steering etc. and to standardize these functions. Efforts have also been directed at attaching ever larger and heavier implements to the tractor and simultaneously increasing the operating speeds so as to raise productivity.